Update docs in PagerManagerInterface

Follow up to #2044435: Convert pager.inc to a service

+++ b/core/lib/Drupal/Core/Pager/PagerManagerInterface.php
@@ -0,0 +1,153 @@
+   *   $where = "status = 1";
+   *   $total = mymodule_select("SELECT COUNT(*) FROM data " . $where)->result();
+   *   $num_per_page = \Drupal::config('mymodule.settings')->get('num_per_page');

@catch:

I'm sure this is copy/pasted but there's no need for $where to be a variable here. Would be good to update the docs to use selectquery or similar (in a follow-up, appreciate this is probably copy and paste from the old example).